Foodborne illnesses are primarily caused by food contaminated with pathogenic microorganisms in the field or during food processing under insanitary conditions. Hence, surveillance of bacterial contamination of fresh produce through the food supply chain is of great importance to the food industry. However, such surveillance is a challenge since the food supply chain is a lengthy trail with many opportunities to cause food contamination. Food products may be cleaned at the harvesting site, transported to a warehouse, re-cleaned, and repackaged several times before reaching retail outlets.
Typical microbiological methods for pathogen detection, such as colony counting, immunoassay, and polymerase chain reaction (PCR), offer very high sensitivities. However, they require pre-analytical sample preparation, which generally includes sample collecting, separating target pathogen cells from food, increasing cell concentration, and achieving analysis volume from bulk samples before detection. These processes are time consuming, resulting in delays in obtaining the screening results. More importantly, food samples have to be delivered to laboratories for culture preparation and analysis. Label-free biosensors are available in today's market. However, they also require sample preparation prior to the actual testing (i.e. sampling from fresh produce, filtration and purification of the collected samples, and injection of the filtered/purified samples into a flow system where a biosensor resides). Due to the complexity of these test procedures and the requirements of expensive equipment and highly trained personnel, current food safety controls mainly rely on control of worker/environment hygiene in the food processing industry, rather than the direct pathogen detection.
Free-standing phage-based magnetoelastic biosensors have been investigated as a label-free wireless biosensor system for real-time pathogen detection. The magnetoelastic biosensor is typically composed of a magnetoelastic resonator that is coated with a bio-molecular recognition element that binds specifically with a target pathogen. Once the biosensor comes into contact with the target pathogen, binding occurs, causing an increase in the mass of the resonator resulting in a decrease in the resonant frequency of the sensor (as well as other characteristic frequencies of the sensor). However, typical exciter/detector coils do not detect magnetoelastic biosensors positioned outside of the coil's interior geometry.
When bacteria cells contaminate a food surface, the distribution of bacteria cells is typically highly non-uniform. The pathogen cells can migrate and move along the food surface and aggregate at regions containing nutrients and water needed to sustain life. By way of example, on a tomato spiked with a high concentration of Salmonella, cells may typically cover almost the whole drop area due to the high concentration of bacteria in the liquid. With a decrease in suspension concentration, the number of Salmonella cells on the surface may decrease and the distribution of cells over the surface may become more non-uniform. As the water of the spiking solution evaporates, the Salmonella cells typically aggregate to areas of residual moisture and form clusters. Therefore, the distribution of Salmonella cells may become highly non-uniform on the tomato surface as the concentration of Salmonella in the spiking solutions decreases. Additionally, variances in the roughness of the tomato, curvature of the tomato surface, punctures, defects, and wounds may contribute to a non-uniform distribution of Salmonella on the tomato surface.